This invention relates to alkali source ionization detectors used in gas chromatography. More specifically, this invention relates to the protection of an alkali source ionization detector from adverse effects resulting by reason of contact with contaminants or reactive agents passing through the detector.
In gas chromatography, a multicomponent sample is separated in the gas chromatographic column and eluted with a carrier gas in order to isolate the components of the original sample as mixture with the carrier gas. This eluate is conducted to any one of a number of detectors, such as flame ionization detectors and the like. One type of ionization detector utilizes an alkali source in the ionization chamber to contact the effluent of the column as it is eluted. Certain compounds, for example nitrogen and phosphorus compounds, produce ionic emissions when contacted with a heated alkali source. The collection of these ionic emissions and the measurement of the resulting ionic current can provide a quantitative indication of these specific compounds.
Recent advances have been made in alkali sensitized detectors for gas chromatography. One significant improvement has been the utilization of electrically heated alkali sources to stabilize the response characteristics of alkali sensitized detectors. Previously, alkali source detectors employed a small hydrogen/air flame for volatilization of an alkali salt and initiation of the reaction mechanism that produced the detector signal. However, since the sensitivity of such early detectors was dependent upon the temperature of the alkali material, these detectors exhibited instabilities due to fluctuating flame temperatures.
An electrically heated alkali source provides for more accurate control of the temperature of an alkali source, resulting in greater stability of the detector. In electrically heated alkali source detectors, a mixture of alkali salts in a silica gel matrix is fused to a heating element, the temperature of which is maintained by a regulated power supply.
At elevated temperatures of 700.degree. C. or higher, alkali sources are easily damaged by certain solvents and derivatizing reagents that are commonly used in gas chromatography. Contact with such solvents can result in damage to the heating element and/or the alkali source. For example, the alkali of the source can be unduly depleted due to the formation of volatile alkali halides when a halogen containing solvent is passed through the cell. Alkali sources can also be destroyed by derivatizing reagents used to prepare the sample such as N,O-bis-(trimethylsilyl)-acetamid (known as "BSA") that are decomposed by the hot source and covered with decomposition products. See Gehrke, et al, U.S. Pat. No. 3,415,864. Therefore, it is desirable to protect the alkali source during the period of passage of such potentially damaging materials.
Typically, halogen solvent and derivatizing agents have a low retention time in a chromatograph column. Accordingly, these materials are among the first materials to exit the column following injection of the sample, and their passage period is predictable.
At present, if potentially damaging materials are to be used, a four-port valve is employed to divert the carrier gas stream away from the detector during the elution of the potentially offensive materials. However in many instances, utilization of a valve is not an ideal solution since it is expensive, introduces dead volume, introduces additionally reactive surfaces which must be protected, and increases the number of components that require attention. Jahnsen et al, U.S. Pat. No. 3,859,209 teaches the use of two multi-port valves to divert organic chemical compounds.
In Giuffrida, U.S. Pat. No. 3,372,994, the use of an alkali-metal salt fused to an electrode and heated by a hydrogen flame is disclosed. The alkali coating allows the detector to selectively emphasize phosphorus-containing organic compounds in mixtures.
Kolb et al, U.S. Pat. No. 3,852,037, addresses the deterioration problem attendant the use of an alkali glass bead which is maintained in a heated, softened state by a hydrogen flame during operation of the detector. Kolb teaches the use of a sensing electrode located above the alkali glass bead, with the electrical conductivity between the bead and the electrode being measured to indicate deterioration of the glass bead. A continuous supply of alkali is made available as the surface area of the glass bead gradually deteriorates.